1. Field of the Invention
The present invention relates to semiconductor wafer handling and processing equipment, and in particular, to an edge grip device for reading an OCR mark on the wafer, wherein the device includes a buffer mechanism that allows a high throughput of wafers through the device.
2. Description of the Related Art
During the fabrication of semiconductor wafers, wafers are transported between the various process tools in the wafer fab in open cassettes or cassettes sealed within a pod such as a standard mechanical interface (SMIF) pod. As wafers move through the various workpiece tools, it is desirable to be able to track and locate a particular wafer at any given time. Moreover, it is desirable to be able to identify a particular wafer during wafer fabrication to ensure that the wafer is subjected only to processes appropriate for that wafer. This wafer tracking is accomplished in part by marking each wafer with an optical character recognition (OCR) mark, or similar indicial mark, which mark is read for each wafer prior to locating a wafer within a processing station. The indicial mark is typically a number or letter sequence etched into an upper and/or lower surface of a wafer near the outer circumference by a laser or other suitable etching means. The indicial mark may alternatively be a bar code or a two dimensional dot matrix at an outer circumference of the wafer.
Before an indicial mark may be read, the mark must first be located. When a wafer is seated within a wafer cassette, the orientation of the wafer to the cassette, and to a tool for extracting and supporting the wafer, is generally unknown. Attempts have been made to align the indicial mark of each wafer at a particular rotational orientation within the cassette. However, because wafers move within a cassette upon handling and transfer of the cassette between processing stations, and because the wafer is often reoriented in a process tool, alignment of the indicial marks prior to transportation has not proved feasible.
In order to locate an indicial mark, wafers are conventionally formed with a fiducial mark, such as a notch, on the outer edge of a wafer. For each wafer being processed, the indicial mark is located in a fixed, known relation to the notch, and by finding the notch, the precise location of the indicial mark may be determined. The notch is typically found by rotating the wafer under a sensor or camera so that the sensor or camera scans the outer edge of the wafer and identifies the position of the notch on the workpiece circumference. Once the notch location is found, the wafer can then be rotated to position the indicial mark under a camera (the same or different than that used to find the notch), and the indicial mark may then be read.
Another reason for rotating the wafer is to determine the radial runout of the wafer. It is important that a wafer be centered when placed to the cassette or process tool. If a wafer is off-center, it may not properly seat on a chuck of a process tool, and/or it may generate particulates by scraping against the sides of the cassette upon transfer of the wafer to the cassette. Therefore, conventionally, it is necessary to determine the radial runout of the wafer and correct it to the center position. The radial runout is a vector quantity representing the magnitude and direction by which a wafer deviates from a centered position with respect to a tool on which the wafer is seated. Once the radial runout has been determined, the wafer may be moved to a center position, or the end effector of the robot may shift to acquire the wafer on center.
Conventionally, a separate operation has been devoted to locating and reading the indicial mark on the wafer, and determining and adjusting for the radial runout of the wafer where it is desired to identify the particular wafers to be processed in that station. This operation is accomplished at a station known as an aligner. Such stations are conventionally located immediately upstream, as a stand alone unit, or as part of each tool in the wafer fabrication process where the indicial mark is to be read.
In conventional aligners, the workpiece handling robot must first transfer the wafer from the cassette to the aligner, the aligner then identifies the radial runout and notch position, the robot or aligner centers the wafer, the OCR mark is read and then the robot transfers the wafer back to the cassette. The robot sits idle while the aligner performs its operations, and the aligner sits idle while the robot transfers the wafers to and from the aligner. Conventional aligner/robot systems therefore have a relatively low throughput, on the order of approximately 300 wafers per hour. This low throughput is significant as the alignment process must be performed at each station where an indicial mark reading is required, and must be performed on each individual wafer at each of these stations. It is known to provide dual armed robots to increase throughput. However, such robots take up additional space within the tool and cleanroom, where space is at a premium. Moreover, the dual armed robots are expensive, require more complicated controls, and are relatively difficult to maintain.
It is also a significant concern for semiconductor manufacturers to avoid scratches and particulates on both sides of a semiconductor. Scratches and particulates on the front surface of the wafer can obviously damage the integrated circuit geometries. However, scratches and particulates on the back side of the wafer can also adversely effect the geometries on the front side of the wafer. In particular, scratches and particulates on the back side of the wafer can raise the top surface of the wafer when supported on a processing chuck. Changing the height of the top surface of the wafer even a slight amount can effect the depth of focus during lithography processes, and thereby adversely effect the geometries formed on the front side of the wafer.
It is therefore an advantage of the present invention to increase the throughput of workpieces processed by an aligner.
It is another advantage of the present invention that the aligner does not sit idle while the robot transfers workpieces to and from the aligner.
It is another advantage of the present invention that the robot does not sit idle while the aligner performs its operations.
It is a further advantage of the present invention to prevent scratches and particulates to the back side of a workpiece.
It is another advantage of the present invention that the aligner supports workpieces at their edge so as not to interfere with a pattern formed on the workpiece.
It is a further advantage of the present invention that the aligner provides passive support for workpieces at their edge without clamping the workpieces to the aligner.
It is another advantage of the present invention to provide an aligner capable of transferring a workpiece to an end effector of a workpiece handling robot in a desired orientation.
It is a still further advantage of the present invention to provide automatic centering of a workpiece on the aligner to allow omission of the conventional radial runout determination step and to allow omission of the sensors used to determine radial runout.
These and other advantages are accomplished by the present invention which in preferred embodiments relates to an edge grip aligner including buffering capabilities. The invention includes chuck arms for receiving a workpiece, rotating the workpiece to identify the notch location, and then positioning the workpiece so that the OCR mark on the workpiece can be read. The chuck arms then hand the workpiece off to buffer arms. Thus, a workpiece handling robot can deliver a new workpiece to the chuck arms and carry the old workpiece away in a single operation, instead of two separate operations as in the prior art.